Ultrasonic transducers are composed, normally, of an electromechanical transducer element, e.g. a piezoelectric element, also called a piezo for short, and a coupling layer, also called a coupling wedge or, not so frequently, a lead-in element. The coupling layer is, in such case, most often manufactured of synthetic material, while the piezoelectric element is composed in industrial process measurement technology usually of a piezoceramic. The ultrasonic waves are produced in the piezoelectric element and led via the coupling layer to the tube wall and from there into the liquid.
Arranged between the piezoelectric element and the coupling layer can be another coupling layer, a so called adapting, or matching, layer. The adapting, or matching, layer, in such case, performs the function of transmission of the ultrasonic signal and simultaneously the reduction of a reflection at interfaces between two materials caused by different acoustic impedances.
Known are also ultrasonic methods and measuring devices for ascertaining concentration and/or size of particles in a fluid as measured medium. Thus, U.S. Pat. No. 6,481,268 shows such a measuring device having at least one ultrasonic transducer. The ultrasonic signal transmitted by the ultrasonic transducer is reflected by particles in the measured medium back to the transducer and there registered as an echo. One embodiment shows two ultrasonic transducers arranged opposite one another on a measuring tube. These transmit and/or receive the ultrasonic signals essentially perpendicularly to the measuring tube axis. Another embodiment shows an individual ultrasonic transducer with a coupling element, which is embodied as a lens, in order to focus the ultrasonic signal into the measuring tube. A measuring of flow is not provided in this document.
In an additional patent of the state of the art, U.S. Pat. No. 5,251,490, an ultrasonic flow measuring device is shown, which ascertains flow through a measuring tube with the Doppler measuring principle. Ultrasonic signals are transmitted in the form of waves, focused by an acoustic lens and reflected on particles in the measured medium. The reflections are greatest in the direct vicinity of the focus. The flow velocity of the liquid is determined from the frequency shift between the in-coupled and reflected waves.
U.S. Pat. No. 5,533,408 discloses an ultrasonic flow measuring device using a combination of the travel-time difference principle and the Doppler principle. In this regard, however, each case has its own sensor. Switching between the sensors of the two measuring principles happens upon the exceeding, or subceeding, of a predetermined measured value.
WO 03/102512 A1 proposes a method for travel-time difference measurement of a flowing fluid, wherein, supplementally, the reflections of the ultrasonic signal on particles in the fluid are ascertained, in order to learn therefrom the concentration of the particles. In this regard, two ultrasonic transducers are usually provided for a travel-time difference measurement, wherein at least one of these ultrasonic transducers is switchable so rapidly from a transmitting state to a receiving state that it can receive the reflections of its transmitted signal on the particles in the fluid, or additional ultrasonic transducers are provided, which are so arranged that they can receive the reflections. For ascertaining the concentration and the size of the particles in the measured medium, the Doppler shift of the moving particles is evaluated. Measurement in motionless measured medium is, thus, not possible.